In printers wherein the print technology used requires continuous engagement of the print head with the print medium during the print mode and disengagement of the print head from the print medium during the return carriage mode, control means are required to bring the print head into contact with the print medium at the beginning of the print cycle and to remove the print head from the print medium during the return cycle.
In one such printing method where a stylus carriage is mounted on a lead screw which on rotation in one direction causes the stylus carriage to move linearly in one direction and on rotation of the lead screw in the other direction causes the stylus to move linearly in the opposite direction, independent mechanical means are used to move and hold the stylus carriage in the print position during the print cycle and to move the stylus carriage away from the print medium during the carriage return mode. Such an arrangement requires an independent mechanical means, a motor to drive the mechanical means, limit switches for detecting end of the print and return carriage cycle as well as complex electronic circuit to sequence and drive the mechanical means.
The present invention contemplates a stylus carraige drive which utilizes a stylus carriage mounted on a lead screw of which the direction of rotation determines the linear direction of stylus carraige movement. In addition, the present invention includes means associated with the stylus carriage which causes automatic angular rotation of the stylus carriage in one or the other direction depending on the rotational direction of the lead screw. This arrangement eliminates the need for an independently controlled mechanical means. Thus, the stylus carriage of the present invention automatically assumes the print position, i.e., the print head in contact with the print medium during rotation of the lead screw in one direction and automatically resumes the non-print position, i.e., the print head out of engagement with the print medium when the lead screw is rotated in the opposite direction during carriage return. A stationary rod is used in conjunction with the stylus carriage drive to limit rotation of the stylus carriage on the lead screw to insure linear movement while at the same time permitting sufficient rotation of the stylus carriage to allow engagement and disengagement of the print head vis-a-vis the print medium.
More specifically friction creating means are associated with the stylus carriage which causes automatic orientation of the print head into and out of print position as the lead screw rotates in one or the other direction. This friction creating means comprises a spring interposed between a nut and the stylus carriage. The nut as well as the stylus carriage has threads which mesh with the threads on the lead screw. The spring causes the threads of the nut and stylus carriage to bear against opposite sides of the threads of the lead screw in opposite directions. The frictional force is self-adjusting inasmuch as wear of the threads is compensated by the spring arrangement, which maintains a constant force.